7.13.9Transformer impedances#

The necessary transformer characteristics can be determined from the transformer type, size, and percentage impedance.

The transformer full load secondary current in amps per phase is:

I fl = kVA x 1000 / (3 x 400)

where

kVA is the transformer rating.

Prospective short circuit current in amps per phase at the secondary terminals is:

I sc = I fl / Z%

where

Z% is transformer percentage impedance.

Transformer impedance in Ohms is:

Z = V / I sc

The assumed phase angle for the impedance is 72.5°, which relates to recommended practice for a transformer with capacity of less than 2 MVA (Standards Australia Handbook HB301 p50).

Transformer resistance in Ohms is:

R = Z cos (72.5°)

Transformer reactance in Ohms is:

X = Z sin (72.5°)

The following transformer data was extracted from Energex Technical Instruction TSD0024c.

From the standard transformer data and the formula provided, the typical transformer characteristics can be calculated as shown here: